Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

Fig. 232.

Fig. 233.

345. Refraction of Light.—When light passes from one medium into another it is bent from its course. This may be illustrated by Fig. 232, in which A B C D is a box, into which a candle, E, is shining. The candle is so placed that the shadow of the side A C falls at D. But let the box be filled with water, and now the shadow is removed to d, as if the candle were at e. This is because the rays of light from the candle, in passing from the air into the water, are bent or refracted so as to take a different direction. Here we have light passing from a rarer into a denser medium. Let us see now how it is when light passes from a denser medium into a rarer. This can be illustrated on Fig. 233. Let the vessel, A B C D, be empty, and let a coin be placed at O. Let the eye, E, be in such a position that a straight line, O G E, from the coin to the eye would strike the side of the vessel a little below the edge, or, in other words, that the edge of the vessel would prevent the eye from seeing it. If now, keeping the eye in this position, water be poured in up to a certain level, say F G, the coin comes into view. This is because light coming from the coin to L is bent into another direction, L E, and the coin therefore appears to the eye to be at K. In this case the refraction is from the perpendicular, P Q, let down through the point L, where the light emerges from the denser into the rarer medium. But When light passes from a rarer into a denser medium the refraction is reversed—it is toward the perpendicular. It is from this refraction of light that a stick partly immersed in water appears to the eye to be broken just at the surface of the water.

Fig. 234.

346. Dawn and Twilight.—The light of the sun, in passing from space into our atmosphere, is refracted. If it were not we should have no daylight preceding the rise of the sun, or twilight after its setting; but light would burst upon the darkness of night at once when the sun appeared above the horizon, and darkness would suddenly succeed to the light of day at sunset. As it is, in the morning the light bends toward us as it strikes across the atmosphere long before we see the sun, and after the sun has disappeared from view at evening its light bends toward us in the same manner. And farther, we really see the sun in the morning before it gets above the horizon, and in the evening after it has gone below it. This may be made clear by Fig. 234. Let the central ball represent the earth. Now as the atmosphere is most dense near the earth, and is rarer as you go outward from the earth, it is represented in the figure as having different layers in order that the operation of the refraction may be more clear to you. The outermost layer is exceedingly rare, and each layer is more dense than the previous one as you go in toward the earth. The light coming from the sun, S, below the horizon into the first layer of air, instead of passing on straight to a, as indicated by the dotted line, bends toward the earth. Then in entering the second layer, instead of passing on to b, it will be bent or refracted still more, as this layer is denser; and so on through all the layers, being refracted in each more than in the previous one. The result is, that as every object is seen in the direction in which the rays from it at length reach the eye, the sun, though really below the horizon, appears to be above it, as represented. The path of light from the sun, as it passes through the air, is a curved line. This is because the air, instead of being of uniform density, lessens in density as we go from the earth. If it were of uniform density the light would be refracted in straight lines, as in the experiments in § 345.

347. Mirages.—Sometimes inequalities occur in the density of the lower portions of the atmosphere, causing, of course, unequal refraction, and producing some strange appearances, termed mirages. For example, at Ramsgate, on the coast of England, there was seen, at one time, as represented in Fig. 235 (p. 268), a ship at such a distance that only her topsails were visible; and above in the air there were two complete images of the ship, the uppermost being erect and the under one inverted. Captain Scoresby, in a voyage to Greenland, saw an inverted image of a ship so well defined that he decided that it was the image of his father's ship, the Fame, which was afterward verified. The ship itself was at that time at a distance of 30 miles. An incident in the early history of the author's place of residence may be cited as an example of mirage. A ship left for England freighted with a valuable cargo, and having on board a large number of the best citizens of the colony. Some time after there was immense excitement in New Haven, because the inhabitants saw, with great distinctness, what they supposed to be this vessel, at only a little distance, apparently sailing against the wind. But it soon disappeared from view, part after part, until the whole was gone. The ship itself was never heard from, and it was supposed at the time that this appearance was a manifestation of Providence for the purpose of informing the colonists what had become of their friends. But what was seen was undoubtedly the reflected image of this or some other ship. It is such appearances as these that have given rise to the stories which have been sometimes told of phantom ships. Mirages are very common in the extensive deserts in hot climates, exhibiting to the eye of the traveler various deceptive appearances, as islands, lakes, etc. In Bonaparte's campaign in Egypt such an appearance caused whole battalions of thirsty soldiers to rush forward, supposing at the moment that a plentiful supply of water was at hand.